Leukocyte rolling and firm adhesion in the microvasculature : functional significance for leukocyte recruitment in inflammation

Leukocyte recruitment to tissues is a key component in the inflammatory process. In order for circulating leukocytes to extravasate, they need to interact with vascular endothelium in several sequential steps. Slow rolling along the endothelial lining is the initial step in this sequence of events, followed by firm adhesion to endothelium and subsequent transmigration across the vessel wall. These interactions are governed by adhesion molecules expressed on the surface of leukocytes and endothelial cells, i.e. leukocyte rolling is mediated predominantly by adhesion molecules of the selectin family whereas firm adhesion is mediated by integrin molecules. This intravital microscopic study aimed at investigating leukocyte/endothelium interactions in inflammation with focus on the functional significance of leukocyte rolling for subsequent steps in the leukocyte extravasation process.

The quantitative relationship between chemoattractant-induced leukocyte firm adhesion and the preceding rolling interaction was investigated in rat mesenteric venules. Systemic treatment with fucoid in, a sulfated polysaccharide which interferes with selectin-receptor function, resulted in a dose-dependent inhibition of leukocyte rolling. Secondary to this effect, chemoattractant-induced firm leukocyte adhesion was reduced in proportion to the decrease in leukocyte rolling. The data demonstrate a close relationship between the extent of leukocyte rolling and the firm adhesive response, and that the initial rolling interaction is a necessary precondition for subsequent steps in the extravasation process. Further studies in the rabbit showed that inhibition of leukocyte rolling with fucoid in profoundly reduced neutrophil infiltration in inflammatory skin lesions, and leukocyte accumulation in the cerebrospinal fluid in an experimental meningitis model, thus illustrating a therapeutic potential of inhibiting leukocyte rolling (e.g. with carbohydrate molecules) in inflammatory disorders.

Heparin and related glycans may, similarly to fucoid in, interfere with leukocyte/endothelium interactions. It was demonstrated that heparin has inhibitory actions on both leukocyte rolling and chemoattractant-induced firm adhesion, and that it may increase local blood flow. Attenuation of the firm adhesive response at low concentrations of heparin could be ascribed mainly to inhibition of selectin-mediated leukocyte rolling, whereas at high concentrations of heparin a direct effect on integrin-mediated firm adhesion was indicated. Because of interference with several microvascular functions, strict dose-dependent responses to heparin treatment were not found, illustrating a complex interplay between local blood flow, leukocyte rolling, and firm adhesion as determinants of leukocyte recruitment to tissues in inflammation.

These interrelationships and the influence of microhemodynamics on leukocyte/endothelium interactions were characterized in more detail. The rolling leukocyte flux was directly proportional to the total leukocyte flux, and to the venular blood flow. Consequently, the rolling/total leukocyte flux fraction did not vary with blood flow and showed no correlation to wall shear rate. Firm leukocyte adhesion evoked by chemoattractant stimulation was quantitatively related to the rolling leukocyte flux, and hence to the venular blood flow, while there was no correlation to the wall shear rate. The data indicate that leukocyte adhesion to the venular endothelium, because of its dependence on the preceding rolling interaction, is proportional to the microvessel blood flow, which emphasizes the importance of tissue hyperemia for leukocyte recruitment in inflammation.

Analysis of the rolling characteristics of polymorphonuclear (PMN) and mononuclear (MN) leukocytes indicated that the great majority of PMN leukocytes passing in venules of the rat mesentery rolled along the endothelial lining, whereas MN leukocytes were rolling only to a small extent under the same basal conditions. In cytokine-activated tissue, on the other hand, MN leukocyte rolling was substantially increased. Differences between the leukocyte subpopulations in their binding interaction with the endothelial selectins and a dependence of MN leukocyte rolling on a4 integrins were suggested to govern the discrete rolling characteristics, and are likely to contribute to the temporal selectivity in the recruitment of leukocyte subclasses to inflamed tissue sites.